An image forming apparatus, which executes the print processing of image data received from a PC or the like, undergoes a change in image output characteristics, depending on an environment in which the device is used, elapsed time and other criteria. For example, in a laser printer, even when the same image data is subjected to print processing, quality of output image changes depending on the consumption of toner, change in photosensitive characteristics of a drum with secular variation and others. In order to correct the image output characteristics, image data subjected to print processing at an image forming apparatus is to be corrected by using correction data. The correction data is data generated on the basis of calibration data obtained at the image forming apparatus for the purpose of correcting the image output characteristics. The calibration data is observed concentration data that is obtained by transferring a test pattern to a photosensitive drum and reading the test pattern with the use of a sensor, for example. When the image data is corrected by using a computer such as a PC, the correction data is generated by the computer on the basis of the calibration data transferred from the image forming apparatus. Then, the image data subjected to print processing is to be corrected by using the generated correction data.